Electroplating is a common process for depositing a thin film of metal or alloy on a workpiece article such as various electronic components. In electroplating, the article is placed in a suitable electrolyte bath containing ions of a metal or alloy to be deposited. The article forms a cathode which is connected to the negative terminal of a power supply, and a suitable anode is connected to the positive terminal of the power supply. Electrical current flows between the anode and cathode through the electrolyte, and metal is deposited on the article by an electrochemical reaction.
In many electronic components it is desirable to deposit the metal film with a uniform thickness across the article and with uniformity of composition. However, the electroplating process is relatively complex and various naturally occurring forces may degrade the electroplating process. Most significantly, the electrical current or flux path between the anode and the cathode should be relatively uniform without undesirable spreading or curving to ensure uniform electrodeposition. Furthermore, as metal ions are depleted from the electrolyte, the uniformity of the electrolyte is decreased and must be suitably corrected to avoid degradation of the electroplating process. Also, debris particles are generated in the chemical reactions which can degrade the metal film on the article upon settling thereon.
Conventional electroplating equipment includes various configurations for addressing these as well as other problems for ensuring relatively uniform electroplating. Suitable circulation of the electrolyte is required for promoting electroplating uniformity, and care is required for properly aligning the cathode and anode to reduce undesirable flux spreading. For example, one type of conventional electroplating apparatus mounts the cathode at the bottom of an electrolyte bathing cell, with the anode being spaced above and parallel to the cathode. Since the article is at a common depth in the cell, the electroplating process is less susceptible to vertically occurring variations in the process due to buoyancy or gravity effects or other convection effects occurring during the process. For example, ion depletion in the electrolyte adjacent to the article will create local currents which will have a common effect along the horizontal extent of the article, but can vary vertically.
Also, in the electrodeposition of magnetic materials, e.g. permalloy, resulting gases are produced in the process which result in bubbles being generated at the article surface. Of course, bubbles are buoyancy driven upwardly, and horizontally positioning the article reduces adverse effects therefrom.
Enhanced uniformity in metal deposition is typically promoted by suitable agitation of the electrolyte in the cell. However, agitation by a unidirectional flow of the electrolyte is typically undesirable since it can cause monotonically decreasing mass-transfer effectiveness along the direction of flow.
Paddles positioned near the article are now commonly used to agitate the electrolyte in the cell. However, because plating is a long process, often occurring 24 hours a day, 7 days a week, the paddle must run continuously. If the paddle stops, the article being plated may have to be discarded, leading to delays in production and expense.
Thus, reliability is a key issue in any wafer fabrication process. The most reliable prior art paddle drive mechanism is a slider crank mechanism. This is attributable to a non-reversing motor, operating in one direction at a single speed, and no need for limit switches or sensors since over travel is impossible.
FIG. 1 depicts a prior art electroplating cell using a slider crank mechanism to drive the paddle, as described in U.S. Pat. No. 3,652,442. As shown, a bath container 110 is surrounded by a magnetic source 112. A cathode 114 is positioned in the bath container 110 and supports the object being plated. An anode 124 is positioned below the level of the fluid 130 in the bath container 110 and above the cathode 114. The bath is agitated during plating by a motor 132 which is connected to a carrier 136 by a linkage 134. When the motor 132 is energized, it pushes and pulls the carrier 136. The carrier 136 in turn moves a base portion 135 continuously in a path back and forth along the length of the cathode 114 and just above the surface of the cathode 114. As a result, a homogenization of the bath solution 130 occurs on the surface of the cathode 114.
The problem with this mechanism is that the base portion 135 follows a sinusoidal velocity profile, i.e., is not constant over the cathode 114. See FIG. 2, which depicts a velocity profile 200 of the base portion 135 across the cathode 114. This type of velocity profile results in uneven film deposition, as the base portion 135 moves faster over certain portions of the object being plated.
As shown in FIG. 2, the velocity of the base portion 135 increases towards its fastest point at about 90 degrees rotation, roughly corresponding to the center of the cathode 114, and decreases as it moves towards 180 degrees and away from the center of the cathode 114. The base portion 135 behaves similarly between 180 and 360 degrees of rotation, where it travels in the reverse direction.
It is more desirable to have a constant velocity over the object being plated. A constant velocity provides the most uniform film deposition.
The most common approach to achieving semi-constant velocity over the wafer is to use programmable motors, such as rotary motors with a worm screw, or linear conversion actuators. These motors provide a generally trapezoidal velocity profile. However, the problem with such drive systems is they are very complex, making them less reliable. This is true particularly in light of the long duty cycles involved in plating.
Programmable motors most often use steps or counts to determine position. If electrical noise causes the drive mechanism to lose (or gain) counts, the mechanism may travel too far, possibly impacting the cell wall, or not far enough and stall. To overcome this problem, the prior art has used limit switches. If a slider drive mechanism triggers a limit switch, the position of the drive assembly is reset to a default position before the drive is re-initiated in the desired mode. However, if this happens more than one or two times, the plating process may be adversely affected and the wafer may need to be discarded.
It is desirable to provide not only high uniform thickness and composition in an electrodeposition article, but also do so in an apparatus capable of reliable operation even when used for high-volume manufacturing. What is needed is a drive mechanism having the reliability of a slider crank with its non-reversing motor and absence of over travel and limit sensing requirements, with the functionality of programmable drive schemes.